you said: " could a simple small green led light be pigtailed to a location of users choosing, when green light comes on, car can start, and leave the monitor under dash, and just pull out if there is a problem? "

sounds like the light at the diesel trucks that goes off when the glow plug has warmed up, just the opposite. takes a few seconds and you should not start until then. and could be located anywhere.
anyway, seems a good idea. simple and practical. lets see what Andre decides.